In the field of refining and petrochemistry and, for example, in catalytic reforming or selective hydrogenation reactions with respect to petrol, it is sometimes appropriate to reduce the activity of the catalysts. An example is nickel catalysts, which are e.g. excellent hydrogenation catalysts for aromatics to such an extent that they can cause runaway reaction during the start-up with new or regenerated catalysts, which may even lead to start-up incidents and to the destruction of the reactor. It is therefore necessary to carry out passivation treatments making it possible to avoid a runaway reaction. These treatments generally consist of irreversibly poisoning by sulphur the most virulent active sites of the nickel existing on the new or regenerated catalyst.
Thus, with more particular regard to refining and hydrotreatment or hydrogenation catalysts based on iron, cobalt, molybdenum, tungsten or nickel, adequate catalysts are marketed and loaded into reactors in the form of oxides, whereas their activated and stable form is the metallic form. Thus, in the prior art, the first stage consisted of reducing the oxides in the metallic state in the reactor (in situ) with hydrogen and then, for obviating the aforementioned disadvantages, in a second stage, there is an in situ catalyst activity reduction by introducing a given quantity of sulphur, generally representing 0.1 to 1.2% by weight of sulphur, based on the catalyst weight. This is generally carried out with the aid of a sulphur-containing compound, such as carbon sulphide, mercaptan, hydrogen sulphide, thiophene compounds, sulphides and disulphides, e.g. dimethyl sulphide DMS or dimethyl disulphide DMDS. In these prior art processes, the reduction with hydrogen (first stage) is performed at a relatively high temperature for a relatively long time (e.g. for reducing nickel oxide to nickel, at approximately 400.degree. C. for 14 hours).